Process for the selective oxidation of gaseous sulfur-containing compounds, hydrogen sulfide in particular, to form elemental sulfur

ABSTRACT

The claimed invention relates to a catalytic process for the selective oxidation of sulfur-containing compounds, in particular hydrogen sulfide, to form elemental sulfur. The catalyst includes a carrier of which the surface exposed to the gaseous phase does not exhibit alkaline properties under the reaction conditions, and a catalytically active material applied thereto or formed thereon. The specific surface area of the catalyst is less than 20 m 2  /g catalyst, with less than 10% of the total pore volume having a pore radius between 5 and 500 Å. A process for preparing the catalyst is also disclosed.

This application is a division of application Ser. No. 07/037,590, filedApr. 13, 1987, now U.S. Pat. No. 4,818,740.

This invention relates to a catalyst for the selective oxidation ofhydrogen sulfide to elemental sulfur; to a process for the preparationof the catalyst; and to a process for the selective oxidation ofhydrogen sulfide to elemental sulfur.

The need to purify gases treated in chemical processes, supplied tocustomers, or discharged to the atmosphere, from sulfur compounds, inparticular hydrogen sulfide, is well known. Indeed, a number ofprocesses for removal of hydrogen sulfide from gas are known.

In some of these processes, hydrogen sulfide is first concentrated bymeans of a liquid absorbent, whereafter the regenerated H₂ S gas isconverted into elemental sulfur, which is not harmful. In certain casesit is possible to omit the first step, i.e., concentrating the hydrogensulfide, and to convert it directly into elemental sulfur. A requisitein many of these cases, then, is that the components in the gas which donot contain sulfur are not reacted. Such a process is called a selectiveoxidation process.

One of the best known methods of converting hydrogen sulfide toelemental sulfur is the so-called Claus process.

The Claus process is carried out in various ways, depending on thehydrogen sulfide content in the gas being treated. In one embodiment aportion of the hydrogen sulfide is combusted to form sulfur dioxide,which then reacts further with the remaining hydrogen sulfide to formelemental sulfur. A detailed description of the Claus process is to befound in R. N. Maddox, `Gas and Liquid Sweetening`; Campbell PetroleumSeries (1977), p. 239-243, and also in H. G. Paskall, `Capability of theModified Claus Process`, publ.: Western Research & Development, Calgary,Alberta, Canada (1979).

In the Claus process, however, the H₂ S is not quantitatively convertedinto elemental sulfur, mainly as a result of the fact that the Clausreaction is not quite completed:

    2H.sub.2 S+SO.sub.2 →2H.sub.2 O+3/n S.sub.n         ( 1).

Accordingly, there are remaining amounts of H₂ S and SO₂. Now, theeffluence of the H₂ S containing residual gas is not permitted, so thatthis has hitherto been combusted, whereby the hydrogen sulfide and othersulfur compounds, and also the elemental sulfur present in the gaseousphase are oxidized to form sulfur dioxide. As environmental requirementsare becoming stricter, this will no longer be permitted, in view of thetoo high emission of sulfur dioxide that would result. It is thereforenecessary to process the residual gas of the Claus installation, theso-called tail gas, further in a so-called tail gas plant.

Tail-gas processes are known to those skilled in the art and described,inter alia, in NL-A-7104155 and in B. G. Goar, `Tail Gas clean-upprocesses, a review`, paper at the 33rd Annual Gas ConditioningConference, Norman, Okla., Mar. 7-9, 1983.

The best known and hitherto most effective process for the treatment oftail gas is the SCOT process. This process is described, for example, inMaddox, `Gas and liquid sweetening` (1977), publ. Campbell PetroleumSeries, p. 280. In this process, the tail gas is passed with hydrogenover a cobalt molybdenum on Al₂ O₃ catalyst, whereby the SO₂ present iscatalytically reduced. The total quantity of H₂ S is subsequentlyseparated in the usual way by liquid absorbtion. This requires previousconversion of SO₂ to H₂ S, as the presence of SO₂ is a very disturbingfactor. One disadvantage of the SCOT process is, therefore, the need ofusing complicated equipment. A further disadvantage is the high energyconsumption needed to regenerate the hydrogen sulfide from theabsorbent.

Another possibility of converting hydrogen sulfide in tail gas toelemental sulfur is the so-called BSR Selectox process, which isdescribed in U.S. Pat. No. 4,311,683.

In that process, the H₂ S containing gas is mixed with oxygen and passedover a catalyst containing vanadium oxides and vanadium sulfides on anon-alkaline, porous, refractory oxidic carrier. The conversion iscarried out at a temperature of between 121° and 232° C.

A major drawback of both the SCOT process and the BSR Selectox processis that in both cases, after the hydrogenation of the sulfur componentspresent to H₂ S, the tail gas must first be cooled to remove the majorpart of the water. In fact, water greatly interferes with the absorptionand oxidation of H₂ S. Owing to the high investments involved, the costof the tail gas treated by these known processes is high.

Another process for the oxidation of H₂ S to elemental sulfur isdescribed in U.S. Pat. No. 3,393,050. According to that publication, thehydrogen sulfide containing gas is passed with an oxidixing gas over asuitable catalyst contained in the tubes of a so-called tubular reactor,with the tubes being externally cooled. The catalyst considered suitableis bauxite, aluminum oxide (gamma alumina) or aluminum silicate asdescribed in U.S. Pat. No. 2,971,824. Apart from the disadvantagesmentioned above, the effectiveness of this process, just as of the otherknown oxidation processes, is insufficient. Thus, for example, the datagiven in U.S. Pat. No. 4,311,683 show that the formation of SO₂ cannotbe avoided, in spite of the low temperature that is used. In view of theratio of H₂ S to SO₂ found in the `product gas` it must be supposed thatthis formation of SO₂ is at least partially connected with the at leastpartial establishment of the Claus equilibrium. Indeed, it is inparticular the occurrence of the following side-reactions whichadversely affect the effectiveness:

1. The continued oxidation of sulfur: ##EQU1##

2. The reverse (or rather reversing) Claus reversible reaction: ##EQU2##In it the sulfur once formed reacts with the water vapour that is alsopresent to form hydrogen sulfide and sulfur dioxide.

3. The so-called sulfation of the catalyst, for example:

    MeO+SO.sub.2 +1/2O.sub.2 →MeSO.sub.4                ( 4).

As a result of this reaction, metal oxides present in the catalyst areconverted into sulfates, whereby the catalytic activity is reduced,sometimes even to a substantial extent.

4. The formation of SO₃ (over certain metal oxides) according to

    SO.sub.2 +1/2O.sub.2 →SO.sub.3                      ( 5)

5. Pore condensation of sulfur formed in the catalyst bed, mainly owingto condensation in the catalyst pores (so-called capillary condensation)which may occur above the sulfur dew point.

The occurrence of the side reactions listed above is partiallydetermined by conditions in practice.

Generally speaking, tail gas contains in addition to elemental sulfur aconsiderable concentration of water vapour, which concentration mayrange between 10 and 40% by volume. This water vapour greatly promotesthe reversing Claus reaction. Substantial removal thereof has evidenttechnological disadvantages, such as the need of an additionalcooling/heating stage, an additional sulfur recovery stage, or ahydrogenation stage followed by a (water removing) quench stage. Aprocess in which the selectivity is not affected by the water content ofthe gas would therefore be highly desirable.

Another important circumstance is that in selective oxidation processes,some excess of oxygen will generally be used, not only to prevent H₂ Sfrom `slipping through`, but also on the ground of considerations ofcontrol technology. It is this very excess of oxygen, however, whichwill give rise to the continued oxidation of the elemental sulfurformed, whereby the effectiveness of the process is adversely affected.

It is an object of the present invention to provide a catalyst for theselective oxidation to elemental sulfur, the use of which substantiallyprevents the side reactions referred to, while the main reaction H₂S+1/2O₂ →H₂ O+1/n S_(n) (6) takes place with a sufficient degree ofconversion and selectiveness.

It is noted in this connection that the term `selective` has a moresharply defined meaning for the catalyst according to the presentinvention, namely, that it virtually only brings about the conversioninto elemental sulfur, by the direct reaction with oxygen. In thatsense, the catalysts of the state of the art, as discussed above, cannotbe called selective.

The catalyst according to the invention comprises a carrier in which thesurface exposed to the gaseous phase does not, under the reactionconditions, exhibit alkaline properties with a catalytically activecomponent applied thereto, with the specific area of the catalyst beingless than 20 m² /g catalyst, and less than 10% of the total pore volumehaving a pore radius of between 5 and 500 Å. The catalyst generallycontains at least 0.1% by weight, calculated on the total mass of thecatalyst, of a material that is catalytically active for the selectiveoxidation of H₂ S to elemental sulfur. The catalytically active materialused is preferably a metal oxide, a mixed oxide of a plurality of metalsor a mixture of metal oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from a consideration of thefollowing detailed description taken in conjunction with the drawings inwhich FIGS. 1-4 are block diagrams of the Claus plant process discussed,respectively, in Examples III-VI.

DETAILED DESCRIPTION

It is known from the literature that alkaline sites on the catalystsurface accelerate the establishment of the equilibrium of the Clausreaction:

    3/n S.sub.n +2H.sub.2 O→2H.sub.2 S+SO.sub.2.

Whereas the hydrogen sulfide formed is rapidly oxidized, sulfur dioxideis stable. Accordingly, when the equilibrium in the Claus reaction isestablished, the gas at the outlet of the reactor will contain sulfurdioxide. As this detracts from the desired selectivity, the catalystaccording to the invention must contain no or substantially no alkalinesites in the surface exposed to the gaseous phase. The presence of suchsites appears from the establishment of the equilibrium in the Clausreaction, which is readily determined by analysis of the gas mixtureexiting from the reactor. According to the invention, therefore, thecarrier is selected so that its surface either contains no orsubstantially no alkaline sites or contains alkaline sites completely orsubstantially completely covered by the active component. Naturally, thedistribution of the active component over the carrier must not bechanged under the reaction conditions to such an extent that alkalinesites are again exposed.

It has been found that, by using the catalyst according to the presentinvention, the undesirable side reactions referred to hereinbefore canbe avoided to a great extent. Without wishing to be bound by any theory,we assume that, in narrow deep pores present in many catalyst carriers,the sulfur formed and diffusing to the outside easily reacts furtherwith oxygen diffusing inwardly to form sulfur dioxide. This mightexplain why this reaction does not occur in a catalyst of the presentinvention, namely, because there are insufficient narrow deep pores. Theabsence or substantial absence of the Claus reaction in a catalystaccording to the invention, too, could be explained by supposing that,in view of the low specific area, there are insufficient active sites,or none at all, to promote the (reversing) Claus reaction.

It is noted that it has not so far been recognized that the occurrenceof the Claus reaction is counterproductive in oxidation processes forhydrogen sulfide, and should therefore be prevented. Thus, for example,in U.S. Pat. No. 3,393,050, cited above, it is observed that sulfurdioxide can be used as the oxidant. This can only be done, however, ifthe catalyst used promotes the Claus reaction and hence the reversingClaus reaction. As under the process conditions, the Claus reactioncannot be irreversible as can be shown thermodynamically, a quantitativeconversion of H₂ S into elemental sulfur is therefore basicallyimpossible.

The use of the catalyst according to the present invention offers asubstantial solution to this problem, because, as explainedhereinbefore, the Claus equilibrium is not established. Accordingly, inselective H₂ S oxidation processes using the catalyst of the presentinvention, the use of oxygen, for example, in the form of air oxygen, isessential.

The conditions of a specific area less than 20 m² /g and a lowmicroporosity as defined for the catalyst according to the presentinvention are highly unusual in the science and technology of catalysis.On the contrary it is generally held that to avoid large volumes ofcatalyst with a desired activity of the catalyst bed, high-area carriersof high porosity are preferable. In the catalyst according to thepresent invention this should be definitely avoided, so that only veryspecific catalysts are suitable for the purposes of the presentinvention.

In the preparation of the catalyst, this must be taken into account intwo respects.

1. In the selection of the starting materials; and in the preparationprocedure.

The preferred starting material will therefore be a carrier whichalready has a low specific area itself. Suitable materials for suchcarriers are metals and metal alloys which are stable under the reactionconditions. Examples are metals such as iron, chromium or nickel oralloys containing one or more of these metals. To bring the catalystcarrier into a suitable form, it may, if necessary, be subjected to apreliminary treatment.

The pore distribution, for example, given by means of a histogram can beused to determine whether the carrier is suitable for the purposes ofthe present invention. During the application of the active component,this specific area must not be essentially increased.

Finally, if desired, a sintering treatment can also be carried out withthe ready catalyst, whereby micropores are sintered away.

The criteria given with regard to the pore distribution and the specificarea are of great importance for the results to be achieved in theprocess, in particular for the selectivity and degree of conversion tobe achieved.

For these reasons, further requirements may be imposed upon the catalystparameters. Preferably the specific area of the catalyst will not exceed10 m² /g catalyst, while especially with a specific catalyst area ofless than 7 m² /g catalyst excellent results are obtained.

The substantial absence of micropores, too, is of importance with regardto the results to be obtained with the catalyst: preferably, no morethan 2% of the total pore volume should occur in the form of poreshaving a radius ranging between 5 and 500 Å.

A particularly suitable carrier is alpha-alumina, but silica whosespecific area satisfies the above requirements, such as, for example,hydrothermally sintered silica, is also suitable for use.

The above carriers are, in principle, ceramic materials which do notgive an alkaline reaction. It is quite possible, however, to use othermaterials which do not give an alkaline reaction under the reactionconditions and are thermostable. Examples are thermostable, non-ceramicmaterials, such as metal mesh structures, surfaces of (incompletely)sintered materials, metal mouldings, packing bodies (`Raschig rings`,etc.) and so on. Highly suitable is a honeycomb structure of highthermal conductivity.

Suitable materials for such carriers are the various metal alloys whichare substantially stable under the reaction conditions. Examples aremetals such as Fe, Cr or Ni or alloys containing one or more of thesemetals.

As stated above, suitable catalytically active materials are a metaloxide, an oxide of a plurality of metals or a mixture of metal oxides.Preferably the catalytically active material used is an iron oxide or anoxide of iron and chromium. A suitable Cr:Fe molar ratio is less than0.5 and preferably between 0.02 and 0.15.

The catalytically active material is present on the carrier in aproportion of 0.05-10% by weight, calculated on the total mass of thecatalyst. The active component is preferably present on the carrier in aproportion of more than 1% by weight, calculated on the total weight ofthe catalyst. Best results were obtained using catalysts in which thiscontent was between 3 and 10% by weight, calculated as the weight of themetal oxide or the mixed oxide of two or more metals and calculated onthe total weight of the catalyst.

It should be emphasized in this connection that we are here concernedwith the active material present on the carrier. In fact, as a result ofa sintering treatment, or from a different method of preparation, aportion of the active material, in particular the metal oxide, maybecome encapsulated within the carrier, for example, when narrow poresare sealed during the sintering treatment. The difference between thisencapsulated or embedded metal oxide and metal oxide present on thecarrier, however, can be readily determined by means of so-calledtemperature programmed reduction (TPR). Details of this measuringtechnique are described in N. W. Hurst, S. J. Gentry, A. Jones and B. D.McNicol Catal. Rev. Sci. Eng. 24(2), 233-309 (1982). The amount of metaloxide present on the carrier and accessible to gases can thus bedetermined.

In principle, the catalysts according to the invention can be preparedusing known methods for the preparation of supported catalysts. Duringsuch preparation, however, in view of the unusually small specific areaand low microporosity of the catalysts according to the invention,specific measures must be taken, in particular, during the preparationthe porosity must not be increased.

Particular care is required in homogeneously applying the catalyticallyactive material to the carrier material, while in addition it should beensured that this homogeneity is maintained during and after the dryingprocedure.

In order to satisfy these requirements, in one possible method ofpreparing the catalyst according to the invention, a powdered porouscarrier material with a low specific area is impregnated in drycondition with a complex solution. This method is known by the name of`incipient wetness impregnation`. Preferably due care is exercisedtherein. Previously the pore volume of the carrier is determined.Subsequently, the carrier is impregnated with a volume of the complexsolution that is equal to or, preferably, slightly less than, the porevolume of the carrier. The complex solution contains the cations of theactive material complexed with an organic molecule in the solution. Tothis solution, an amount of a viscosity-increasing compound, such ashydroxy-ethyl cellulose, may be added. By impregnating the carriermaterial with this complex solution by the incipient wetness method alow-area catalyst is obtained, to which the active material is appliedhighly homogenously and in which the microporosity has not increased ascompared with the starting carrier material.

During the drying procedure, the temperature should be increased veryslowly to maintain homogeneity. Electron micrographs, porosimetrymeasurements, B.E.T. measurements and reactor experiments show that thecatalysts satisfy the requirements with regard to texture.

In case a catalyst on a non-ceramic carrier is used, the catalyticallyactive material can be applied to the carrier in known manner.

The invention also relates to a process for selectively oxidizingsulfur-containing compounds, in particular hydrogen sulfide, toelemental sulfur, using the catalyst according to the present invention.

In this process, hydrogen sulfide is oxidized directly to elementalsulfur by passing a hydrogen sulfide containing gas together with anoxygen containing gas over the catalyst at elevated temperature.

It is observed that, for optimum results, not only is the structure ofthe catalyst determinative, but so are the process variables. Inparticular, the temperature, contacting period, and excess of oxygenselected are of importance for the oxidation. The oxidation process iscarried out by adding to the hydrogen sulfide containing gas, using aknown per se ratio regulator, oxygen or an oxygen containing gas in sucha quantity that the molar ratio of oxygen to hydrogen sulfide rangesbetween 0.5 and 5.0 and preferably between 0.5 and 1.0.

The process according to the present invention can be used for theselective oxidation of all gases which contain sulfur containingcompounds, in particular hydrogen sulfide. Examples of processes inwhich the oxidation according the invention can be suitably applied arethe processes as described in European patent application 91551,European patent application 78690, and U.S. Pat. No. 4,311,683.

The process according to the invention is excellently suitable foroxidizing gas containing no more than 1.5% of H₂ S, as it is thenpossible to use a normal, adiabatic reactor.

In the oxidation, the inlet temperature of the catalyst bed is selectedto be in excess of 150° C., and preferably in excess of 170° C. Thistemperature is partially determined by the requirement that thetemperature of the catalyst bed must be above the dewpoint of the sulfurformed. Using known per se measures, the maximum temperature in thecatalyst bed is maintained below 330° C. and preferably below 300° C.When the H₂ S content is higher than 1.5%, it may be necessary to takemeasures to prevent that, owing to the reaction heat released, thetemperature in the oxidation reactor becomes too high. Such measurescomprise, for example, the use of a cooled reactor, for example, atubular reactor in which the catalyst is contained in a tube surroundedby a coolant. Such a reactor is known from European patent 91551. It isalso possible to use a reactor incorporating a cooling element.Furthermore the gas being treated may be returned to the reactor inletafter cooling, whereby an extra dilution of the gas to be oxidized isachieved, or alternatively, the gas to be oxidized may be distributedover a plurality of oxidation reactors, simultaneously distributing theoxidation air over the various reactors. In a particular embodiment ofthe process according to the invention, the catalyst is used as a fluidmedium in a fluid-bed reactor, preventing short-circuiting by applyingone or more apertured plates. In this way optimum heat transfer isensured. In another particular embodiment the catalyst is used in theform of solid, for example, honeycomb structures of high thermalconductivity whereby an undue rise in temperature of the catalyst isalso suitably prevented.

The process according to the invention can be used with particularadvantage for the selective oxidation of the hydrogen sulfide containingresidual gases from a Claus plant. Apart from the very high selectivityof the catalyst according to the invention, the important advantage isobtained that the removal of water prior to the oxidation is no longerneeded. When the residual gases are oxidized, using the processaccording to the present invention, these are preferably first passedthrough a hydrogenation reactor containing, for example, a cobaltmolybdenum containing catalyst, and in which all sulfur components arehydrogenated to form hydrogen sulfide.

According to a variant of the process according to the invention, theselective oxidation stage using the catalyst according to the inventionis combined with a subsequent hydrogenation stage, followed byabsorption of hydrogen sulfide, all this as described in European patentapplication 71983. In this way, 98% of the sulfur compounds present isremoved in the section preceding the hydrogenation, so that thehydrogenation stage and the absorption mass are not unduly loaded. Inthis way, sulfur recovery percentages up to 100 can be reached. In avariant of this process the hydrogenation stage is followed by anotherselective oxidation according to the invention, instead of theabsorption stage, whereby an overall sulfur recovery percentage ofbetween 99.5 and 99.8 is obtained.

Furthermore, the process according to the invention is particularlysuitable for desulfurizing, for example, fuel gases, refinery gases,biogas, coke furnace gas, gaseous effluents from chemical plants such asviscose factories or gases burnt off at gas and/or oil winning sites.Furthermore, the catalyst according to the invention can be used as aprotective layer in Claus reactors to protect the normal Claus catalystfrom being sulfated. The catalyst then serves to cause small amounts ofoxygen to react with H₂ S to form sulfur, and sulfate formation in theClaus catalyst is thus prevented.

When, in the process according to the invention, the sulfur vapourcontaining gas from the selective oxidation stage is passed over a bedin which the sulfur is removed by capillary absorption, the sulfurrecovery percentage is increased to virtually 100.

The invention is illustrated in and by the following examples.

EXAMPLE I

4.784 g Fe(NO₃)₃.9H₂ O and 0.540 g Cr(NO₃)₃.9H₂ O are dissolved in alittle demineralized H₂ O. 3.9 g EDTA (ethylene diamine tetraaceticacid) is dissolved in a little 25% NH₃ solution. The two solutions areadded to form a slurry. To this slurry 25% NH₃ solution is added untilall of the slurry is dissolved and the pH has reached a value of 7. Thecolour of the solution is then blood red. Subsequently the total weightof the solution is made up with demineralized H₂ O to 20 g. Finally, 0.4g HEC (hydroxyethylcellulose) is carefully added to this solution.

20 g alpha-Al₂ O₃ (Fluka, powder 6.5 m².g⁻¹) is impregnated with theabove solution. The total mass is stirred well for 15 minutes.Thereafter, the resulting catalyst is allowed to dry at room temperaturefor 48 hours and then at 120° C. for another 48 hours. The catalystcontains 4.5% by weight of Fe₂ O₃ at 0.5% by weight of Cr₂ O₃.Subsequently, catalyst pellets are pressed at 160 MPa. Of these pellets,a sieve fraction is made containing particles between 850 μm and 1000μm, and this sieve fraction is calcined in air at the desiredtemperature. After calcination at 500° C. for 18 hours, the N₂ B.E.T.area of the catalyst is 6.94 m².g⁻¹ mercury. Porosimetry proves thatless than 1% of the total pore volume is in pores having a radius lessthan 500 Å.

EXAMPLE II

A quartz reactor tube with a diameter of 1.2 cm is filled with 7.8 ml ofa catalyst prepared according to Example I and calcined at 500° C. for18 hours. On top of this catalyst a 4 cm-layer of quartz pieces of thesame dimensions as the catalyst pellets is placed. A gas mixtureconsisting of H₂ S, O₂, He and possibly H₂ O is passed through thereactor from the top downwards. The sulfur formed is condensed in thebottom part of the reactor at a temperature of 140° C. The H₂ O formedis then collected in a desiccant (P₂ O₅ on SiO₂).

The results of the experiments are listed in the following table. Allmeasurements have been carried out at a space velocity of 1000 Nm³ /h.m³catalyst.

It follows from the data specified in the table that the catalyst islittle sensitive to an excess of O₂. Furthermore, 30% by volume of H₂ O,present in the feedstock gas gives not a negative, but rather a positiveeffect on the total sulfur production.

    ______________________________________                                         %)(vol.H.sub.2 S                                                                   ##STR1##                                                                               (Vol. %)H.sub.2 O                                                                      (°C.)T                                                                      version (%)H.sub.2 S con-                                                              vity(%)selecti-sulfur                                                               (%)yieldsulfur                    ______________________________________                                        1    0.6       0       210   98      94    92                                                        230  100      94    94                                                        250  100      91    91                                                        270   91      85    77*                                1    0.8       0       210   99      94    93                                                        230  100      92    92                                                        250  100      88    88                                                        270  100      82    82                                 1    0.6      30       210   93      94    87                                                        230   95      96    92                                                        250  100      94    94                                                        270  100      93    93                                 1    0.8      30       210   98      93    91                                                        230  100      95    95                                                        250  100      95    95                                                        270  100      90    90                                 ______________________________________                                         *This deviating value is obtained as a result of there being a relative       oxygen deficit for the H.sub.2 S oxidation.                              

COMPARATIVE EXAMPLE I

Of a CRS-31 catalyst, marketed by Rhone Poulenc, and used in both theClaus reaction and direct oxidation processes of hydrogen sulfide, asieve fraction was made of particles having a diameter of between 850and 1000 μm. Using 7.8 ml of this sieve fraction, the reactor is filledin the same way as described in Example II.

The catalyst has a total area of 146.5 m² g⁻¹. Porosimetric measurementsshow that 90% or more of the total pore volume is in pores having aradius less than 500 Å.

The results of the reactor experiments are listed in the followingtable. All measurements have been made at a space velocity of 1000 Nm³/h.m³ catalyst. The feedstock gas consisted of 1% by volume of H₂ S,0.6-0.8% by volume of O₂, 0-30% by volume of H₂ O and He.

It follows from the data listed in the table that the CRS-31 catalyst issensitive to an excess of O₂. Also, the presence of 30% by volume of H₂O in the feedstock gas has a very negative effect on the total sulfuryield.

    ______________________________________                                         %)(Vol.H.sub.2 S                                                                   ##STR2##                                                                               (Vol. %)H.sub.2 O                                                                      (°C.)T                                                                      version (%)H.sub.2 S con-                                                              vity(%)selecti-sulfur                                                               (%)yieldsulfur                    ______________________________________                                        1    0.6       0       210  98       95    93                                                        230  98       93    91                                                        250  96       91    87                                                        270  95       88    84                                 1    0.8       0       210  98       79    77                                                        230  98       78    76                                                        250  97       80    78                                                        270  95       79    75                                 1    0.6      30       210  95       90    86                                                        230  92       87    80                                                        250  88       82    72                                                        270  83       78    65                                 1    0.8      30       210  97       71    69                                                        230  95       68    65                                                        250  92       63    58                                                        270  88       59    52                                 ______________________________________                                    

COMPARATIVE EXAMPLE II

A double-walled vessel having a capacity of 1.5 l is filled with 500 mldemineralized H₂ O, in which 30 g gamma-Al₂ O₃ (Harshaw Al-3-31-P) issuspended. The suspension is kept at a temperature of 38° C. by means ofa water bath.

37.875 g of Fe(NO₃)₃.9H₂ O and 3.750 g of Cr(NO₃)₃.9H₂ O are dissolvedin 500 ml of demineralized H₂ O. The pH is adjusted to 1.3 with HNO₃.The solution last mentioned is pH-statically injected into thesuspension of gamma-Al₂ O₃ at a rate of 0.5 ml.min⁻¹. As a base, a 0.2MNH₃ solution is used.

After filtration, the catalyst is washed thrice with demineralized H₂ O.Thereafter the catalyst is dried at 120° C. for 24 hours. Subsequently,catalyst pellets are pressed at 80 MPa. Of these tablets a sievefraction is made with particles of between 850 μm and 1000 μm. Thissieve fraction is calcined in air at 500° C. for 18 hours.

The catalyst contains 19.6% by weight of Fe₂ O₃ and 1.9% by weight ofCr₂ O₃.

The N₂ B.E.T. area is 253.6 m².g⁻¹. Porosimetry shows that 73% or moreof the total pore volume is in pores having a radius less than 500 Å.

7.8 ml of this catalyst is used for reactor experiments. The reactor isfilled as specified in Example II.

The results of the reactor experiments are summarized in the followingtable. All measurements have been made at a space velocity of 1000 Nm³/h.m³.

    ______________________________________                                         %)(Vol.H.sub.2 S                                                                   ##STR3##                                                                               (Vol. %)H.sub.2 O                                                                      (°C.)T                                                                      version (%)H.sub.2 S con-                                                              vity(%)selecti-sulfur                                                               (%)yieldsulfur                    ______________________________________                                        1    0.65     0        220  99       84    83                                                        260  98       86    84                                                        270  99       86    85                                 1    0.85     0        230  99       66    65                                                        250  99       68    67                                 ______________________________________                                    

EXAMPLE III

As indicated in FIG. 1, the Claus reaction is carried out in a Clausplant having two catalytic stages.

Supplied to the thermal stage are a Claus gas containing 90% by volumeof H₂ S, corresponding to 90 kmoles/h, 5% by volume of CO₂ and 5% byvolume of H₂ O, as well as 45 kmoles/h of O₂ as air oxygen. The H₂ S %by volume in the residual gas after the second catalytic stage is 0.58,the SO₂ content therein is 0.29% by volume. After the conversion of allsulfur components to H₂ S in a hydrogenation stage, using a reducing H₂/CO gas at 280° C., the gas, which contains a considerable quantity ofwater vapour is oxidized using the catalyst according to the inventionas described in Example I.

The H₂ S percentage by volume in the output gas from the hydrogenationstage is 0.9, which corresponds to 2.5 kmoles/h; the H₂ O contenttherein is 35.8% by volume, corresponding to 98.9 kmoles/h. To theselective-oxidation stage, 2.0 kmoles/h O₂ is supplied in the form ofair oxygen, corresponding to an O₂ :H₂ S ratio of 0.8, i.e., an excessof oxygen of 60%. The gas to the selective-oxidation reactor is cooledto 180° C. In the selective-oxidation reactor, the H₂ S is fullyconverted at a bed temperature of 230° C.

The oxidation efficiency to elemental sulfur is 90%; the remainder isconverted into SO₂. The resulting total sulfur recovery percentage inthe overall system is 99.6. The effluent from the selective-oxidationstage is passed to the chimney via an after-burner.

EXAMPLE IV

Using the equipment illustrated in FIG. 2, the Claus reaction is carriedout in a Claus reactor, comprising two catalytic stages. Supplied to thethermal stage are a Claus gas containing 90% by volume of H₂ S,corresponding to 90 kmoles/h, 5% by volume of CO₂ and 5% by volume of H₂O, as well as 44.22 kmoles/h of O₂, in the form of air oxygen. Inaddition, a quantity of gas of 10 kmoles/h containing 0.77 kmole/h ofSO₂, from a dry-bed absorption stage, is circulated. In the residual gasfrom the Claus plant, the H₂ S concentration is 0.58% by volume, whichcorresponds to 1.58 kmoles/h; the SO₂ concentration is 0.29% by volume,corresponding to 0.79 kmole/h; and the H₂ O content therein is 35.8% byvolume, corresponding to 98.9 kmoles/h. The residual gas is oxidizedfurther, using the catalyst according to the invention as described inExample I. To the selective oxidation stage, 0.94 kmole/h O₂ is suppliedin the form of air oxygen, which comes down to an O₂ :H₂ S ratio of 0.6,an oxygen excess of 20%. The gas to the selective-oxidation reactor isheated to 195° C. In the selective-oxidation reactor, the H₂ S is fullyconverted at a bed temperature of 230° C. The oxidation efficiency toelemental sulfur is 90%; the balance is converted into SO₂. Aftercondensation of the sulfur, the gas is mixed with a reducing H₂ /CO gas,heated to 280° C., and then supplied to the hydrogenation reactor.

All SO₂ in the gas and the remaining sulfur components are convertedinto H₂ S. The gas is then passed over the dry-bed absorption stage. Thereactors of the dry-bed absorption stage are filled with absorption massas described in European patent application 71983. The H₂ S is absorbedto the absorption mass and in this way removed from the gas. The gasfrom the absorption stage flows to the after-burner and thence to thechimney. In order to maintain the system at the desired pressure duringthe regeneration of the absorption mass, a minor gas stream of 10kmoles/h is bled off and recycled to the Claus plant.

In total, a sulfur recovery percentage of 99.9 is obtained.

EXAMPLE V

Using the apparatus as shown in FIG. 3, the Claus reaction is carriedout in a Claus plant having two catalytic stages. Supplied to thethermal stage are a Claus gas containing 90% by volume of H₂ S,corresponding to 90 kmoles/h, 5% by volume of CO₂ and 5% by volume of H₂O, as well as 45 kmoles/h of O₂ in the form of air oxygen. The H₂ Spercentage by volume in the residual gas after the second catalyticstage is 0.58, which corresponds to 1.56 kmoles/h and the SO₂ contenttherein is 0.29% by volume, corresponding to 0.78 kmole/h. Using thecatalyst according to the invention as described in Example I, the H₂ Sin the gas is selectively oxidized to sulfur in the presence of aconsiderable concentration of water vapour.

The H₂ O content is 35.8% by volume, which corresponds to 98.9 kmoles/h.To the selective oxidation stage, 0.94 kmole/h of O₂ is supplied in theform of air oxygen, which comes down to an O₂ :H₂ S ratio of 0.6, anoxygen excess of 20%. The gas to the selective-oxidation reactor isheated to 195° C. In the selective oxidation reactor, the H₂ S is fullyconverted at a bed temperature of 230° C.

The oxidation efficiency to elemental sulfur is 90%. The balance isconverted to SO₂. After condensation of the sulfur formed, the gas ismixed with a reducing H₂ /CO gas, heated to 280° C., and subsequentlysupplied to the hydrogenation reactor. All SO₂ in the gas and theremaining sulfur components are converted into H₂ S.

Subsequently, the gas is re-supplied to a selective oxidation stage, inwhich H₂ S is oxidized to sulfur, using the catalyst according to theinvention as described in Example I. The H₂ S percentage by volume inthis gas is 0.39, which corresponds to 1.1 kmoles/h; the H₂ O contenttherein is 35.9% by volume, which corresponds to 100 kmoles/h.

To the second selective oxidation stage, 0.88 kmoles/h of O₂ is suppliedin the form of air oxygen, which corresponds to an O₂ :H₂ S ratio of0.8, an excess of oxygen of 60%.

The gas to the second selective-oxidation reactor is cooled to 250° C.In the second-selective oxidation reactor, the H₂ S is again fullyconverted at a bed temperature of 230° C.

The oxidation efficiency of H₂ S to elemental sulfur in the second stageis 90%; the balance is converted to SO₂.

As a result a total sulfur recovery percentage of 99.8 is obtained inthe overall system. The effluent gas is discharged to the chimney viathe after-burner.

EXAMPLE VI

Using the apparatus as shown in FIG. 4, a quantity of 5000 Nm³ /h fuelgas is supplied to a catalyst according to the present invention, asdescribed in Example I. The fuel gas contains 63% by volume of CH₄, 31%by volume of CO₂, 1% by volume of H₂ S and 5% by volume of H₂ O.

To the selective-oxidation stage, 30 Nm³ /h of O₂ is supplied in theform of air oxygen, corresponding to an O₂ :H₂ S ratio of 0.6, and anoxygen excess of 20%. The gas to the selective-oxidation reactor isheated to 180° C. The H₂ S is fully converted at a bed temperature of240° C. The oxidation efficiency to elemental sulfur is 92%; the balanceis converted to SO₂. After condensation of the sulfur formed, the gas isdischarged. The sulfur recovery percentage is 92.

EXAMPLE VII

Incoloy 825 metal chips are boiled for 2 hours in a 5% NH₄ OH solution.Thereafter the chips are thoroughly washed in demineralized water andsubsequently calcined in air at 500° C. for 24 hours.

0.39 g H₄ EDTA (ethylene diamino tetraacetic acid) is dissolved in about20 ml H₂ O and adjusted to pH7 with NH₃. To this solution 0.46 gFe(NO₃)₃.9H₂ O is added, and the pH is re-adjusted to 7 using NH₃. Now0.125 g of HEC (hydroxyethyl cellulose) and 0.25 g agar are added. Theweight of this impregnation liquid is made up to 50 g by means of H₂ O.The solution is boiled for 1 minute.

36.1 g of the washed and calcined metal chips are impregnated with 10 mlof the above impregnation liquid. Thereafter the catalyst is dried invacuo at 20° C. for 64 hours. Finally, the iron complex is convertedinto active iron oxide by calcining in air at 500° C. for 24 hours. Theiron oxide load of the catalyst is then 0.125% by weight.

EXAMPLE VIII

A cylindrical quartz reactor (diameter 12 mm) is filled with 7.8 ml of acatalyst prepared in accordance with Example VII. A gas mixtureconsisting of hydrogen sulfide, oxygen and helium is passed through thereactor from the top downwards. To ensure proper admixture of thereactants in the reactor, a layer 4 cm thick of quartz lumps has beenplaced on top of the catalyst. The sulfur formed is condensed in thebottom portion of the reactor at a temperature of 140° C. The resultingwater is collected afterwards in a desiccant. This desiccant isphosphorus pentoxide on silica. The remaining gas stream, containingsulfur dioxide, helium and possibly hydrogen sulfide and oxygen, isquantitatively analyzed on a gas chromatograph.

The results of the experiments are collected in the following table. Allmeasurements have been made at a space velocity of 1150 Nm³ /h.m³catalyst. The input concentrations of hydrogen sulfide and oxygen were1.0% by volume and 0.8% by volume, respectively.

    ______________________________________                                        temperature                                                                   reactor (°C.)                                                                      activity (%)                                                                             selectivity (%)                                        ______________________________________                                        250          43.3      95.2                                                   282          93.7      85.5                                                   303         100.0      79.2                                                   315         100.0      71.0                                                   ______________________________________                                    

The activity is defined as the conversion of hydrogen sulfide. Theselectivity is the quantity of sulfur formed from the converted hydrogensulfide.

What we claim is:
 1. A process for the selective oxidation of sulfurcontaining compounds to form elemental sulfur, comprisingpassing ahydrogen sulfide containing gas together with an oxygen containing gas,at a temperature of at least 150° C., over a catalyst which comprisesacarrier of which the surface exposed to the gaseous phase does notexhibit alkaline properties under the reaction conditions, and acatalytically active material applied thereto or formed thereon, thespecific surface area of the catalyst being less than 20 m² /g catalyst,with less than 10% of the total pore volume being constituted by poreshaving a pore radius between 5 and 500 Å; said catalyst beingineffective to establish the Claus equilibrium.
 2. A process as claimedin claim 1, wherein the molar ratio of oxygen to hydrogen sulfide ismaintained between 0.5 and
 1. 3. A process as claimed in claim 1,wherein H₂ S is the only sulfur containing compound which is oxidized tosulfur.